The invention relates to a discharge device, especially for a gas laser, having a discharge chamber through which a gas flows at a high velocity. The gas discharge is a steady-field discharge between an anode and a cathode opposite the anode as the main electrodes, which extend on either side of the flow of the gas. The device has a pre-ionizing electrode which is covered with dielectric material and fed with an alternating current, and has a length corresponding approximately to that of the cathode and anode. This pre-ionizing electrode is situated ahead of the cathode and/or the anode in the direction of flow of the gas, and it is connected to one of the main electrodes through a low impedance in regard to the pre-ionizing alternating current.
U.S. Pat. No. 4,488,309 discloses a discharge device of this kind, in the form of a gas laser. This gas laser has an anode extending in the direction of flow, which defines one side of the gas discharge channel for the gas; opposite this anode are a number of pin-like single cathodes disposed athwart the gas flow, each connected by high-ohmic impedances to the direct-current source. About in the middle between the anode and the single cathodes there is disposed a rod-like pre-ionizing electrode which has a conductive core enveloped in a dielectric coating. The pre-ionizing electrode is connected to an alternating current source. Also, between the anode and the ground potential there is a capacitance, and between the output of the power source and the anode an inductance, which serve as energy accumulators for pulsing. The capacitance and inductance must be designed for a high pulse energy so as to keep the continuous output of the direct-current power supply low. With this arrangement a stable DC glow discharge is produced, which can easily be switched and pulsed, i.e., has a good starting performance; also, the voltage of the DC discharge is lowered. In this circuit the pre-ionizing discharge burns exclusively between the pre-ionizing electrode and the anode, since high input impedances are in the cathode circuit. This has the result that the gas volume in the discharge chamber is pre-ionized but irregularly, and the power requirement for the purpose is relatively high. The cost of the power supply is high, since the production of a high-frequency alternating current power source is disproportionately expensive in comparison to the DC power supply.
German Federal Patent 28 56 328, to which U.S. Pat. No. 4,449,220 corresponds (incorporated herein by reference), discloses another discharge device for a gaseous medium, in which the auxiliary discharge is a steady-field pre-ionization produced by a plurality of pin-like pre-ionizing electrodes projecting from the wall of the discharge chamber. Power on the order of 40% of the total input power is required for the auxiliary discharge. This is given additional importance by the fact that the individual secondary electrodes are provided with high input impedances for the stabilization of the additional discharge, and these cause a considerable power loss. Additional complication develops if the discharge is to be switched rapidly, since either two independent power supplies must be used or the main discharge part and the auxiliary discharge part have to be definitely separated from on another.
WO 82/01281 discloses one embodiment of pre-ionizing arrangements of this kind. In particular it describes the shape of single added electrodes for DC pre-ionization, and T-shaped hooks, U-shaped hooks and L-shaped hooks extending into the discharge chamber are given as possible forms. This pre-ionizing arrangement requires a great amount of power.